app, pdb: "Add Layer Mask" dialog asks whether to edit mask immediately.

To be merged after GIMP 3.0.0 release, because we are in feature freeze.

@aryeom was demonstrating how annoying current "add mask" behavior is when creating a lot of mask and when they are right from initialization (e.g. because based on selection). You need to click the layer thumbnail back each and every time to get out of "edit layer mask" mode.

Now it will be an option to adapt to people's workflow.